Preheat your oven to 200°C/400°F/gas 6. Wash the sweet potatoes and bake them, in their skins, in the preheated oven until cooked and soft. Allow to cool (leaving the oven on), then peel and mash them.
Bash up 1 teaspoon of the coriander seeds and the chillies in a pestle and mortar. Stir with...